This is a late style alarm, with the revised MST 427 - sharing some overt styling cues with the Memovox. The MST427 replaced the earlier MST417 in 1955, with revised alarm cancel and hammer mechanism. The calibre numbers are not visibly marked - except under the dial. The way you can tell the difference at a glance is the extra hole by the Roamer text by the hammer in the 417. The Roamer Alarm with either calibre is one of the hardest Roamer watches to find, although no production figures remain - it was not a high number. I think it is best said in the words of a former Roamer manager - "we didn't make many of those".
The roamer alarm calibre is quite sophisticated, with a single winding and setting crown for all alarm and time functions. The single pusher is to cancel the alarm. When winding, the alarm is wound when the crown is rotated clockwise, and the mainspring is wound when rotating anti-clockwise. The alarm sets when the alarm spring is wound. The second position on the stem adjusts the alarm pointer, and the third position sets the hands. The winding mechanism features a rocker as in the Vulcain cricket that alternating barrels, depending on the direction of the winder.
Associated Patents
1951 hammer mechanism CH295436 - MST 417 1952 sounding back CH304446 1955 revised hammer mechanism CH321585 - MST 427
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